Low temperature coefficient alloy



Dec. 17, 1963 B. L. AVERBACH 3,

LOW TEMPERATURE COEFFICIENT ALLOY Filed Jan. 13. 1961 s Sheets-Sheet 1 F/Gi/ Dec. 17, 1963 B. 1.. AVERBACH 3,114,662

LOW TEMPERATURE COEFFICIENT ALLOY Filed Jan. 13, 1961 5 Sheets-Sheet 2 'Dec. 17, 1963 B. L. AVERBACH 3,114,662

LOW TEMPERATURE COEFFICIENT ALLOY Filed Jan. 13, 1961 3 Sheets-Sheet 3 United States Patent 3,114,662 LOW TEMPERATURE COEFFICIENT ALLOY Benjamin L. Averbach, Belmont, Mass, assignor, by mesne assignments, to Weinschel Engineering Co. Inc, Gaithersburg, Md., a corporation of Delaware Filed Jan. 13, 1961, Ser. No. 82,593 7 Claims. (Cl. 14S-142) This invention relates to a nickel-iron alloy having a low coeificient of thermal expansion and to a process for making the alloy. More specifically, it relates to a low expansion nickel-iron alloy having a low oxygen content, as well as controlled amounts of other impurities. The excellent thermal characteristics are due in part to the composition of the alloy and in part to a novel heat treatment thereof.

Prior to the present invention, there have been a number of commercially available alloys having relatively low thermal expansion coethcients. Almost invariably, they are iron alloys containing approximately somewhere between 30 and 40 percent nickel. The literature indicates that a great deal of effort has been expended in attempts to improve the thermal characteristics of these alloys, principally by modification in the various con stituents thereof. However, the best coefiicient consistently obtainable has been about 1.0 part per million per degree C. Some alloys have exhibited lower coeificients for short periods of time, but they are unstable in their expansion properties and subject to dimensional changes with age. Others are seriously affected by temperature shock. There are a number of applications where lower coefiicients, together with stability, are required. For example, resonant cavities, used in wave meters and like devices, must have highly stable d mensions in order to provide the desired accuracy of operation.

Accordingly, it is a principal object of my invention to provide an improved alloy having a low thermal coefiicient of expansion.

More specifically, it is an object of the invention to provide an alloy having a lower thermal coetficient than eretofore consistently obtainable.

Another object of the invention is to provide an alloy of the above type having a low thermal coefficient of expansion over a wide temperature range.

A further object of the invention is to provide an alloy of the above type Whose dimensions and thermal coefiicient of expansion are stable and relatively unaffected by thermal shock.

Yet another object of the invention is to provide an alloy of the above type available in commercial quantities and at reasonable cost.

A still further object of the invention is to provide a process for making an alloy of the above type.

Another object of the invention is a process of the above type providing a high yield of alloy having improved thermal expansion characteristics.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others, and the composition possessing the features, properties, and the relation of constituents, which are exemplified in the following detailed disclosure,

3,114,652 Patented Dec. 17, 1963 "ice and the scope of the invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings, in which:

FIGURE 1 is a photornicrograph showing the microstructure of an alloy embodying the principles of my invention,

FIGURE 2 is a photomicrograph showing the microstructure of an alloy having excessive grain boundary oxidation,

FIGURE 3 shows the microstructure of an alloy having approximately the permissible limit of grain boundary oxidation,

FIGURE 4 shows the microstructure of an alloy exhibiting grain boundary separation, and

FIGURE 5 shows the microstructure near the surface of a bar of the alloy of FIGURE 1 subsequent to forging.

All of the figures represent a magnification of 200x, with the exception of FIGURE 4, in which the magnification is 250x.

The alloy incorporating the features of my invention consists mainly of nickel and iron, with the various impurities controlled within tolerance limits set forth below. In particular, I have found that an important factor in thermal expansion of previous alloys of this. type is excessive oxygen, both distributed throughout the alloy and localized in the form of grain boundary oxidation. Reduction in oxygen content has been largely instrumental in bringing about production of heats in which the thermal expansion coefficient may be conservatively stated as being uniformly below 0.3 p.p.m./ C. In fact, practically all exhibit coefficients below 0.1 p.p.m./ C. over a range of 40 C. to C.

The process for making the alloy includes operation in a vacuum furnace to prevent absorption of various atmospheric constituents. In also includes a novel twostep heat treatment, which has proven to be an important factor in reduction of the expansion coeflicient as well as stability of the various characteristics of the alloy.

More specifically, the alloy contains the following constitutents and their proportions of the total weight, as determined by conventional chemical analysis:

Nickel 35.736.7%.

Carbon 0.020.l0%.

Oxygen 0.005% max.

Nitrogen 0.005% max.

Manganese 0.02% max.

Cobalt 0.15% max.

Other impurities 0.10% max, with. no individual impurity exceeding 0.03

Iron Balance.

I have found that if the oxygen limit is placed at 0.0025 percent, the coefiicient of expansion is almost uniformly 0.0 p.p.m./ C.

The oxygen limit given above does not include all of the oxygen appearing as grain boundary oxidation, since it appears that the oxides in the grain boundaries do not show up completely in chemical analysis. I have found that this type of oxidation has a substantial effect on the coeificient of thermal expansion, and, therefore, the alloy should be comparatively free from it. Grain boundary oxidation can be conveniently ascertained by means of microscopic examination of the grain boundaries, as will be apparent from FIGURES l, 2 and 3. These figures are photomicrographs of alloy specimens taken after grinding and then etching with a solution comprising 4 grams CuSOI, in 20 cc. each of HCl and H and cc. HNO The solution selectively etches along oxidized grain boundaries, leaving voids which are readily distinguished in the photographs.

Thus, the specimen shown in FIGURE 1 exhibits little or no grain boundary oxidation. There are minor inclusions of foreign matter, as shown, for example, by the specks i0, 12 and 14, but these had no measurable effect on the characteristics of the material. The specimen shown in FIGURE 2, on the other hand, has excessive grain boundary oxidation, as indicated by the lines 16, abundant over the entire surface of the specimen. In measurements of the coefiicients of expansion of the heats whose grain boundary oxidation is shown in these figures, the one represented in FIGURE 1 was found to have a coeflicient of 0.1 p.p.m./ C., whereas the heat represented in FIGURE 2 has a coefficient well in excess of the usable range.

FIGURE 3 illustrates a border line case in which, while the alloy is substantially free of grain boundary oxidation, there is a sufiicient amount of such oxidation to show up in a photomicrograph. The oxidized grain boundaries, some of which are designated by the numeral 18, are not nearly so numerous as in FIGURE 2, and, furthermore, the oxidation at these boundaries is not so intensive as in the case of the unacceptable alloy.

Within limits, carbon appears to be an antidote for oxygen in the alloy. Evidently, it reacts with the oxygen to form compounds which, in small quantities, do not appreciably affect the expansion characteristics. Therefore, if the oxygen content is found to consistently approach either of the above limits, i.e., on either chemical analysis or microscopic examination, the carbon content may be increased, within the specified tolerance, to ensure maintenance of a low coefficient of expansion.

The ingots may be hot-worked, for example, rolled or forged. However, the temperature of the alloy should not exceed 2200 F. Otherwise, there is separation of grain boundaries, with a resulting deleterious effect on the thermal expansion characteristics. Also, deep oxidation is likely to take place if this temperature is exceeded. Following hot-working, the material should be free from cracks and excessive mill scale. The mill scale, which is an indication of the amount of penetration of oxidation into the interior of the alloy, increases with the amount of time it is maintained at elevated temperatures, in a normal atmosphere, for hot-working. After hot-working, the surface of the material should be machined to remove the scale and also the exterior layers of metal which have become oxidized and further, in many cases, subjected to some grain boundary separation.

FIGURE 5 shows the grain structure close to the surface of the alloy of FIGURE 1 after forging. There is heavy grain boundary oxidation, as well as evidence of some grain boundary separation close to the surface of the bar (the top of the photomicrograph), as indicated, for example, by the heavy line 20 in this figure. It is noted that the lines diminish in number and intensity at short distance from the surface. The surface material is subject to excessive thermal expansion and its presence has a substantial effect on the thermal characteristics of the entire bar. Therefore, removal of the surface layers substantially enhances the over-all expansion characteristic.

FIGURE 4 illustrates the effect of excessive heat in hot-working, even in the deep interior of the material. There is a substantial amount of grain boundary separation, as indicated by the lines 22. In this particular case,

the separation was suflicient to increase the coefiicient of thermal expansion substantially beyond the values obtainable With the present invention. This specimen also had an excessive amount of included foreign matter, as indicated by the large number of specks 24 distributed throughout.

After hot-working and surface removal, the material should be heat treated in a two-part operation, the first step consisting of heating to a temperature of 2000: 10 F. in a non-oxidizing atmosphere. For example, a nitrogen atmosphere (lamp grade), a salt bath or a neutral atmosphere of air and gas may be used. Alternatively, an H atmosphere may be used to advantage in view of its ability to react with oxygen present in the metal. This serves to reduce the oxygen content as well as prevent formation of new oxides. The material should remain at this temperature for an hour and then it should be quenched in cold water or brine. Next, it is reheated to 1400110 F. for an hour in a neutral environment and once again quenched. It is preferable that the maximum transverse dimension of the article being heat treated be one half inch or less. If the thickness is greater, the quenching is not so effective, since the rate of cooling of the innermost portions is reduced below the elTective value thereof.

The first heating and quenching of the alloy serves primarily to homogenize it, i.e., thoroughly combine the iron and nickel. When heat treatment is not performed, portions of these constituents are often found separately in isolated pockets which exhibit the expansion characteristic of iron and nickel rather than the desired low coefficient of the alloy.

The heat treatment also serves to get the impurities into solution, rather than in precipitate form where they have a more adverse effect on the coemcient of expansion. This is particularly true of graphite, which precipitates upon slow cooling of the metal. The elevated temperatures eliminate the graphite, and the quenching prevents its reprecipitation.

The temperatures to which the alloy is exposed during heat treatment may vary somewhat outside the specified limits without ill effect, particularly with certain proportions of the various constituents. However, in any case, they should be suificient to provide complete alloying of the iron and nickel and substantial solution of impurities. Both separation of iron and nickel and precipitation of impurities can be detected by microscopic analysis.

After heat treatment, final machining, grinding, etc., may be performed. Then the article is preferably aged at a temperature of 200i5 F. for 48 hours to achieve dimensional stability. There is ordinarily some structural growth during this relatively short aging period, and final finishing operations may then be performed to correct for this.

The alloy may be illustrated by the following examples, to which, however, the invention is not limited:

An alloy was prepared having the following proportions of ingredients:

After preparation, including heat treatment in the manner specified above, the alloy was found to have a thermal coeificient of expansion of 0.0 p.p.m./ C. over the range of +10 to +50 C.

An alloy was prepared having the following proportions of ingredients:

After preparation according to the specified process, the coefiicient of thermal expansion was found to be 0.1 p.p.m./ C.

(III) In this case, the alloy had the following proportions of ingredients:

Nickel 36.00%. Carbon 0.07%. Oxygen 0.0011%. Nitrogen 0.0003%. Manganese 0.01%.

Other impurities Not measured. Iron Balance.

After preparation in the specified manner, the alloy was found to have a thermal coefficient of expansion of 0.0 p.p.m./ C.

An alloy was prepared with the following proportions of ingredients:

Percent Nickel 36.4 Carbon 0.052 Oxygen 0.00089 Nitrogen 0.0001 Manganese 0.01 Other impurities 0.083 Iron Balance After preparation, the alloy was found to have a coefiicient of thermal expansion of 0.0 p.p.m./ C.

An alloy was prepared with the following proportions of ingredients:

Nickel 36.12%. Carbon 0.058%. Oxygen 0.002%. Nitrogen 0.0005%. Manganese 0.003%. Other impurities Not measured. Iron Balance.

After preparation, the alloy was found to have a coefficient of thermal expansion of 0.0 p.p.m./ C.

In each of the above examples, the alloy was found to be substantially free from grain boundary oxidation in accordance with the criteria set forth above, as illustrated in the drawings.

Thus, I have described an improved nickel-iron alloy having a low coefiicient of thermal expansion. More specifically, the coefficient is materially less than that previously commercially obtainable. By limiting the proportions of various impurities, particularly that of oxygen, I have obtained an improvement of better than an order of magnitude. The oxygen appears in two diiferent ways, subject to detection by different techniques. The first is standard chemical analysis, which measures the proportions given in the above tables and examples. Observation of the microstructure is used to determine grain boundary oxidation, which also has an adverse effect on the coefficient of expansion.

I have also described a novel process for the alloy. The process includes melting the ingredients in an inert atmosphere, preferably under vacuum conditions, removal of surface oxidation after hot-working and heat treatment in the manner specified above.

It Will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained and, since certain changes may be made in carrying out the above method and in the composition set forth without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.

I claim:

1. A process for making an alloy having a stable low coefficient of thermal expansion, said process comprising the successive steps of (a) alloying from 35.7 percnet to 36.7 percent nickel with iron,

(12) maintaining said nickel-iron alloy substantially free of oxygen and of grain boundary oxidation,

(0) heating said alloy to a temperature of substantially 2000 F. in a non-oxidizing atmosphere,

(d rapidly quenching said heated alloy in a liquid,

(e) heating said alloy to a temperature of substantially 1400 F. in a non-oxidizing atmosphere, and

(f) rapidly quenching said heated alloy in a liquid.

2. The method defined in claim 1 in which the article being treated has a maximum thickness of one-half inch.

3. A process for making an alloy having a stable low coefiicient of thermal expansion, said process comprising the successive steps of (a) alloying the following proportions of ingredients:

Nickel 35.7 to 36.7%.

Carbon 0.02 to 0.10%

Oxygen 0.005% max.

Nitrogen 0.005% max.

Manganese 0.02% max.

Cobalt 0.15% max.

Other impurities 0.1% max., no impurity exceeding 0.03

Iron Balance.

(b) heating said alloy to a temperature of 2000 F. 1rlilO" in a non-oxidizing atmosphere for at least one our,

(0) rapidly quenching said heated alloy with a cold liquid,

(d) reheating said article to a temperature of 1400 F.

l-1 *-10 in a non-oxidizing atmosphere for at least one our,

(e) rapidly quenching said reheated alloy with a cold liquid, and

(f) aging said alloy at a temperature of 200 F. t5

for at least 48 hours. 4. A method as defined in claim 3 for making an article wherein prior to said heating and quenching steps (a) said alloy is hot Worked to provide a desired shape,

and

(b) the outer layers of said article are removed to remove mill scale and material having appreciable grain boundary oxidation.

5. A process for making an alloy having a low coefficient of thermal expansion, said process comprising the successive steps of:

(a) alloying from 35.7 percent to 36.7 percent nickel with iron,

(b) maintaining said nickel-iron al-loy substantially free References Cited in the file of this patent of oxygen and of grain boundary oxidation, UNITED STATES PATENTS (c) heating SEllCl alloy to a temperature of substant1ally I k 2000 F. ina non-oxidizing atmosphere, Jones 1932 (d) rapidly quenching said heated alloy in a liquid, 5 2050387 Scott 1936 (e) heating said alloy to a temperature of substantiaily 2089044 Thomas 1937 1400 F. in a non-oxidizing atmosphere, OTHER REFERENCES mPldlY quenchmg heated m a hqmd and Nilvar: Alloy Digest, Filing Code: Fe-8 Iron Alloy,

(g) aging said alloy by maintaining it at a temperature of substantially 200 F. for at least 48 hours. 10 6. The method defined in claim 1 in which said heated alloy is quenched in Water.

7. The method defined in claim 1 in which said heated alloy is quenched in brine.

August 1955, published by Engineering Alloys Digest, Inc., Upper Montclair, New Jersey.

Metals Handbook, 1948 edition, pages 599, 603, and 1211, published by the American Society for Metals. Metals Handbook, 1948 edition, pages 600-602. 

1. A PROCESS FOR MAKING AN ALLOY HAVING ASTABLE LOW COEFFICIENT OF THERMAL EXPANSION, SAID PROCESS COMPRISING THE SUCCESSIVE STEPS OF (A) ALLOYING FROM 35.7 PERCENT TO 36.7 PERCENT NICKEL WITH IRON, (B) MAINTAINING SAID NICKEL-IRON ALLOY SUSTANTIALLY FREE OF OXYGEN AND OF GRAINBOUNDARY OXIDATION, (C) HEATING SAID ALLOY TO A TEMPERATURE OF SUBSTANTIALLY 2000*F. IN A NON-OXIDIZING ATMOSPHERE, (D) RAPIDLY QUENCHING SAID HEATED ALLOY IN A LIQUID, (E) HEATING SAID ALLOY TO A TEMPERATURE OF SUBSTANTIALLY 1400*F. IN A NON-OXIDIZING ATMOSPHERE, AND (F) RAPIDLY QUENCHING SAID HEATED ALLOY IN A LIQUID. 